Since fuel economies and the need to protect the environment have become priorities, it has become desirable to produce elastomers with good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions that are useful in the manufacture of various semi-finished products for tires, such as underlayers, calendering or sidewall rubbers, or treads, and are useful in obtaining tires with improved properties, such as reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed. First of all, some solutions essentially concentrate on the use of elastomers that have been modified by agents such as coupling, starring or functionalizing agents, where carbon black is used as the reinforcing filler, with the aim of obtaining a good interaction between the modified elastomer and the carbon black. It is generally known that in order to obtain the optimum reinforcement properties imparted by a filler, the filler should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogenously as possible. Such conditions can only be realized insofar as the filler has a good ability firstly to disagglomerate and be incorporated in the matrix during mixing, and on the other hand to be dispersed homogeneously in the elastomer.
It is well known that carbon black has such abilities, which is generally not true of white fillers. For reasons of mutual attraction, white filler particles have a tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and hence its reinforcing properties to a substantially lower level than that which would be theoretically possible if all of the (white filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. In addition, these interactions tend to increase the consistency of the rubber compositions in the uncured state and to make them more difficult to work (“processability”) than in the presence of carbon black.
However, interest in rubber compositions reinforced with white filler was greatly revived with the publication of European Patent Application EP 0 501 227, which discloses a sulfur-vulcanizable diene rubber composition, reinforced with a special precipitated silica of the highly dispersible type, which makes it possible to manufacture a tire or tread with substantially improved rolling resistance, without adversely affecting the other properties, in particular the properties of grip, endurance and wear resistance.
Additionally, Europe Patent Application EP 0 810 258 discloses a diene rubber composition reinforced with another particular white filler, in this case a specific alumina (Al2O3) with high dispersibility, from which tires or treads that show an excellent compromise between contradictory properties can also be made.
Although the use of these specific, highly dispersible silicas or aluminas as reinforcing fillers has certainly reduced the difficulty of processing the rubber compositions that contain them, they are still more difficult to process than-rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent, the function of which is to provide the connection between the surface of the white filler particles and the elastomer, while also facilitating the dispersion of this white filler within the elastomeric matrix.
The term “coupling agent” (white filler/elastomer) is understood, as is known, to mean an agent capable of establishing a sufficient chemical and/or physical bond between the white filler and the diene elastomer; such a coupling agent, which is at least bifunctional, has, for example, a simplified general formula “Y—W—X” in which:                Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the white filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the hydroxyl (OH) surface groups of the white filler (for example, surface silanols in the case of silica);        X represents a functional group (“X” function) capable of bonding physically and/or chemically to the diene elastomer, for example, by means of sulfur atom; and        W represents a hydrocarbon group connecting Y and X.        
In particular, coupling agents should not be confused with simple agents for covering the white filler which, as is known, may comprise the Y function that is active with respect to the white filler, but do not have the X function which is active with respect to the diene elastomer.
Coupling agents, notably (silica/diene elastomer) agents, have been described in numerous documents, the best known being bifunctional alkoxysilanes.
Thus, in French Patent Application FR 2 094 859, it was proposed to use a mercaptosilane coupling agent for the manufacture of tire treads. It was quickly discovered, and is today well known, that the mercaptosilanes, and in particular γ-mercaptopropyltrimethoxysilane or γ-mercaptopropyltriethoxysilane, are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the high reactivity of the —SH functional groups, which very rapidly results in premature vulcanization during the preparation of the rubber composition in an internal mixer. This is also known as “scorching,” which results in very high Mooney plasticity values and results in rubber compositions which are virtually impossible to work and process industrially. To illustrate the impossibility of using such coupling agents bearing —SH functional groups and the rubber compositions that contain them industrially, the documents FR 2 206 330 and U.S. Pat. No. 4,002,594 may be referred to.
To overcome this drawback, it has been proposed to replace these mercaptosilanes by polysulfurized alkoxysilanes, in particular bis-trialkoxy(C1–C4)silylpropyl polysulfides such as those described in numerous patents or patent applications (such as, for example, FR 2 206 330, and U.S. Pat. Nos. 3,842,111, 3,873,489, 3,978,103 or 3,997,581). These polysulfurized alkoxysilanes are now generally regarded as the products which, in the case of vulcanizates filled with silica, give the best compromise with regard to scorching resistance, processability and reinforcing power. Among these polysulfides, mention should be made of bis-3-triethoxysilylpropyl tetrasulfide (abbreviated to TESPT), which is the (white filler/diene elastomer) coupling agent currently recognized as the most effective, and is therefore the one most widely used today in rubber compositions for tires, notably those compositions intended for tire treads. TESPT is commercially available, for example, under the name “Si69” from the company Degussa. However, this product has the known disadvantage that it is very costly and usually has to be used in relatively large amounts. (See, for example, U.S. Pat. Nos. 5,652,310; 5,684,171; and 5,684,172.)